1. Field of the Invention
The present invention relates to a method of marking so-called code marks on the surfaces of surgical instruments and other metal-made products and surgical instruments with code marks marked by using the method, and more particularly to the method of enabling small optically readable code marks to be correctly read by a reader.
2. Description of the Related Art
Optically readable code marks such as bar codes are becoming more commonly used in various fields. With the recent increase in the amount of information to be encoded in code marks, typical bar codes with which information can be encoded in only one direction only have become unable to provide a sufficient space for encoding.
Therefore, so-called binary code marks, by which information can be encoded in vertical and horizontal directions, have been put into use. As examples of this type of code mark, "carla code", "vericode" and "data code" have come into general use. Among them, the data code is disclosed in U.S. Pat. No. 4,939,354 and is designed such that individual cells of a grid pattern divided in vertical and horizontal directions are colored black or white to thereby create a code mark.
Meanwhile, the kinds of products provided with code marks have also been increasing recently. For example, code marks are put on surgical equipment, such as surgical instruments, for the purpose of inventory control of said equipment in hospitals. Since many surgical instruments are small in size, code marks are required to be correspondingly small on the order of, e.g., 2.times.2 mm in size. Furthermore, bar codes are also becoming increasingly used in books and other publications. In order not to mar the finish of bindings or covers, code marks put on books are ideally as small as possible.
In the case of the data code, a mark is taken in as an image by a camera, and the mark image in black and white is converted into a matrix comprised of 0's and 1's, thereby decoding the data encoded in the mark. For correct decoding of the data, therefore, black and white mark elements must be precisely and distinctly discriminated without errors and be displayed as an image. Then, even if a mark is small, a camera has to be able to take in such a small mark with correct discrimination between black and white.
However, if labels prepared with printed code marks are stuck to surgical instruments, in an attempt to put the data code on surgical instruments, the labels would be torn off or the code marks would become blurred, making it impossible to read the code soon, because the surgical instruments are washed and disinfected whenever used. In such an application field, therefore, it is desirable to mark code marks directly on the surface of the surgical instruments. This equally applies to other metal-made products for different uses where durability of the code mark is a problem, in addition to surgical instruments.
When a code mark is engraved on, e.g., surgical instruments by using a laser, recesses and projections on the metal surface are taken in the form of an image by a camera and are discriminated respectively as black and white elements of the mark. In other words, unlike a mark printed in black and white, a mark in the form of recesses and projections on a metal surface, which is not originally colored, has to be recognized as a black and white image. It is thus required that, when recesses and projections on the metal surface are photographed by a camera, they are correctly discriminated and reproduced to be black and white in the form of an image.
However, it is quite difficult to precisely mark small binary code marks, such as the carla code, vericode and data code, by the conventional methods of engraving or electrolytic erosion.
When a mark in the form of recesses and projections is marked on the metal surface, the reflecting condition of light is different depending upon differences in the finish of the metal surface (e.g., mirror surface, pear-skin or hairlines). Accordingly, such a mark must be marked in such a manner that it can be correctly read without being affected by the reflecting condition of light as much as possible.
Moreover, the shapes of surgical instruments are not fixed and portions in which marks are marked are not always flat. Accordingly, marks are also required to be marked in such a manner that they can be correctly read regardless of the shape of the surface.